1. Field of the Invention
The present invention relates to a charged particle beam writing apparatus, and an apportionment method of irradiation time of charged particle beams for multiple writing, and for example, to an apparatus and method for calculating the irradiation time of each beam in multiple writing.
2. Description of Related Art
The lithography technique that advances microminiaturization of semiconductor devices is extremely important as being a unique process whereby patterns are formed in the semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. For forming a desired circuit pattern on such semiconductor devices, a master or “original” pattern (also called a mask or a reticle) of high accuracy is needed. Thus, the electron beam (EB) writing technique, which intrinsically has excellent resolution, is used for producing such a highly precise master pattern.
FIG. 16 is a schematic diagram explaining operations of a variable shaped electron beam writing or “drawing” apparatus. As shown in the figure, the variable shaped electron beam writing apparatus operates as described below. A first aperture plate 410 has a quadrangular opening 411 for shaping an electron beam 330. A second aperture plate 420 has a variable-shape opening 421 for shaping the electron beam 330 having passed through the opening 411 of the first aperture plate 410 into a desired quadrangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the opening 411 is deflected by a deflector to pass through a part of the variable-shape opening 421 of the second aperture plate 420, and thereby to irradiate a target object or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g., the x direction) during the writing. In other words, a quadrangular shape that can pass through both the opening 411 and the variable-shape opening 421 is used for pattern writing in a writing region of the target object 340 on the stage continuously moving in the x direction. This method of forming a given shape by letting beams pass through both the opening 411 of the first aperture plate 410 and the variable-shape opening 421 of the second aperture plate 420 is referred to as a variable shaped beam (VSB) method.
In the electron beam writing apparatus, when performing multiple writing, a shot time (irradiation time) per shot is calculated by dividing a total beam irradiation time (total writing time) of a plurality of times of shooting the same position by the multiplicity. Conventionally, the method of adding an entire remainder (fraction) obtained by dividing the total beam irradiation time (the total writing time) by the multiplicity to one shot has been employed. On the other hand, as the generation of the writing apparatus advances, the multiplicity tends to increase, which causes a problem in that there is a divergence (imbalanced nature) between the irradiation time of a shot to which the remainder was added and the irradiation time of each of other shots.
As other technique relevant to the shot time of each shot, the following is disclosed: Specifically, when a basis dose based on which a pattern is formed is defined to be Ds, the first time writing and the second time writing are performed each using a dose Ds/4 without correcting the dose, and the third time writing and the fourth time writing are performed in corrected state based on a correction dose Dc, using a dose (Dc−Ds)/2+Ds/4 (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 10-261557).
As described above, a problem exists in that there is a divergence (imbalanced nature) between the irradiation time of a shot to which the remainder was added and the irradiation time of each of other shots. For example, when the irradiation time is defined according to a gray level value, the irradiation time of the shot to which the remainder described above was added is longer than the irradiation time of each of other shots by up to (multiplicity−1 gray level). For example, when the multiplicity can be set up to 1024, a remainder of up to 1023 gray levels may be generated. For example, when a pattern whose total irradiation time is 2047 gray levels is written by the multiplicity of 1024, there is a possibility of a case in which a specific number-th writing is performed by an irradiation time of 1024 gray levels, and writing of each of other 1023 times is performed by the irradiation time of 1 gray level. This causes a problem of degradation of writing precision due to heat, for example. Even if the same irradiation dose is used, the formed dimension is thicker when irradiation is performed at a time compared with the case of irradiation performed separately. Therefore, a problem occurs in that if a large dose difference is generated between each time of multiple writing, impermissible influence is given to the formed dimension. Moreover, for example, in the case of simultaneous radiation of a plurality of beams (for example, a case of multiple beam writing), since a following shot cannot be executed until a shot of the longest irradiation time has been completed, a problem of lowering the writing processing speed is generated. Furthermore, in the case of multiple writing, since writing is performed while shifting the position of a writing unit region, such as a stripe region and a sub field region, and further, since, in each writing unit region, multiple writing is repeatedly performed without shifting the position, it is desirable not to make a divergence of total irradiation time between writing unit regions as much as possible for suppressing the imbalanced nature of irradiation time between shots.